Sensory feedback systems and methods for guiding users in virtual reality environments

ABSTRACT

Sensory feedback (“chaperoning”) systems and methods for guiding users in virtual/augmented reality environments such as walk-around virtual reality environments are described. Exemplary implementations assist with preventing collisions with objects in the physical operating space in which the user acts, among other potential functions and/or uses.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Provisional Application Ser. Nos.62/075,742, filed on Nov. 5, 2014, and 62/126,695, filed on Mar. 1,2015, and the contents of both of those applications are hereinincorporated by reference in their entirety.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

The disclosure relates generally to sensory feedback systems and methodsfor guiding users in virtual/augmented reality environments such aswalk-around virtual reality environments and for assisting withpreventing collisions with objects in the physical operating space inwhich the user acts.

2. General Background

Various augmented and/or virtual reality systems and/or environments areknown. One current generation of desktop virtual reality (“VR”)experiences is created using head-mounted displays (“HMDs”), which canbe tethered to a stationary computer (such as a personal computer(“PC”), laptop, or game console), or self-contained. Such desktop VRexperiences generally try to be fully immersive and disconnect theusers' senses from their surroundings.

Collisions with physical objects when using a walk-around virtualreality system are currently solved in certain situations by eitherhaving a second person in the operating space (a “chaperone”) guidingthe user, and/or by providing physical hints (e.g., by placing a thickcarpet on the floor that ends some distance from the adjacent walls).

It is desirable to address the current limitations in this art.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example, reference will now be made to the accompanyingdrawings, which are not to scale.

FIG. 1 is an exemplary diagram of a computing device that may be used toimplement aspects of certain embodiments of the present invention.

FIG. 2 is an exemplary diagram of a human user wearing a head-mountedvirtual reality apparatus comprising optical receivers and sensors thatmay be used to implement aspects of certain embodiments of the presentinvention.

FIG. 3 is an exemplary diagram of a transmitter/receiver configurationin an optical positional tracking system that may be used to implementaspects of certain embodiments of the present invention.

FIG. 4 is an exemplary diagram of a head-mounted virtual reality displaywith four optical receivers that may be used to implement aspects ofcertain embodiments of the present invention.

FIG. 5 depicts an exemplary display according to certain embodiments, inwhich soft bounds are visible on the floor as a translucent polygon andhave been defined by the player, and in which hard bounds are visible asa grid of glowing lines that indicate the position of a physical wall ina player's real-life space.

FIG. 6 depicts an exemplary display according to certain embodiments, inwhich two in-headset views of hard-bounds chaperoning systems are shown,warning a user of an impending collision, and in which the depictedgrids indicate real-world wall positions.

FIG. 7 depicts an exemplary display according to certain embodiments,depicting an example of using soft bounds data for placement of otherelements in a game scene that help frame the player's experience.

FIG. 8 depicts an exemplary display according to certain embodiments,depicting an example of rendering a pattern on the walls of a virtualspace to visually alert a user as to the location of physical boundariesin the real operating space surrounding the user.

FIG. 9 depicts an exemplary display according to certain embodiments,depicting another example of rendering a pattern on the walls of avirtual space to visually alert a user as to the location of physicalboundaries in the real operating space surrounding the user.

FIG. 10 depicts an exemplary display according to certain embodiments,depicting another example of rendering a pattern on the walls of avirtual space to visually alert a user as to the location of physicalboundaries in the real operating space surrounding the user.

FIG. 11 depicts an exemplary display according to certain embodiments,depicting another example of rendering a pattern on the walls of avirtual space to visually alert a user as to the location of physicalboundaries in the real operating space surrounding the user.

FIG. 12 depicts an exemplary display according to certain embodiments,depicting an example of rendering a pattern on the floor of a virtualspace to visually alert a user/developer as to the location of physicalboundaries in the real operating space surrounding the user.

DETAILED DESCRIPTION

Those of ordinary skill in the art will realize that the followingdescription of the present invention is illustrative only and not in anyway limiting. Other embodiments of the invention will readily suggestthemselves to such skilled persons, having the benefit of thisdisclosure, and the general principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the present invention. Thus, the present invention is notintended to be limited to the embodiments shown, but is to be accordedthe widest scope consistent with the principles and features disclosedherein. Reference will now be made in detail to specific implementationsof the present invention as illustrated in the accompanying drawings.The same reference numbers will be used throughout the drawings and thefollowing description to refer to the same or like parts.

The data structures and code described in this detailed description aretypically stored on a computer readable storage medium, which may be anydevice or medium that can store code and/or data for use by a computersystem. This includes, but is not limited to, magnetic and opticalstorage devices such as disk drives, magnetic tape, CDs (compact discs)and DVDs (digital versatile discs or digital video discs), and computerinstruction signals embodied in a transmission medium (with or without acarrier wave upon which the signals are modulated). For example, thetransmission medium may include a communications network, such as theInternet.

FIG. 1 is an exemplary diagram of a computing device 100 that may beused to implement aspects of certain embodiments of the presentinvention. Computing device 100 may include a bus 101, one or moreprocessors 105, a main memory 110, a read-only memory (ROM) 115, astorage device 120, one or more input devices 125, one or more outputdevices 130, and a communication interface 135. Bus 101 may include oneor more conductors that permit communication among the components ofcomputing device 100. Processor 105 may include any type of conventionalprocessor, microprocessor, or processing logic that interprets andexecutes instructions. Main memory 110 may include a random-accessmemory (RAM) or another type of dynamic storage device that storesinformation and instructions for execution by processor 105. ROM 115 mayinclude a conventional ROM device or another type of static storagedevice that stores static information and instructions for use byprocessor 105. Storage device 120 may include a magnetic and/or opticalrecording medium and its corresponding drive. Input device(s) 125 mayinclude one or more conventional mechanisms that permit a user to inputinformation to computing device 100, such as a keyboard, a mouse, a pen,a stylus, handwriting recognition, voice recognition, biometricmechanisms, and the like. Output device(s) 130 may include one or moreconventional mechanisms that output information to the user, including adisplay, a projector, an A/V receiver, a printer, a speaker, and thelike. Communication interface 135 may include any transceiver-likemechanism that enables computing device/server 100 to communicate withother devices and/or systems. Computing device 100 may performoperations based on software instructions that may be read into memory110 from another computer-readable medium, such as data storage device120, or from another device via communication interface 135. Thesoftware instructions contained in memory 110 cause processor 105 toperform processes that will be described later. Alternatively, hardwiredcircuitry may be used in place of or in combination with softwareinstructions to implement processes consistent with the presentinvention. Thus, various implementations are not limited to any specificcombination of hardware circuitry and software.

In certain embodiments, memory 110 may include without limitationhigh-speed random access memory, such as DRAM, SRAM, DDR RAM or otherrandom access solid state memory devices; and may include withoutlimitation non-volatile memory, such as one or more magnetic diskstorage devices, optical disk storage devices, flash memory devices, orother non-volatile solid state storage devices. Memory 110 mayoptionally include one or more storage devices remotely located from theprocessor(s) 105. Memory 110, or one or more of the storage devices(e.g., one or more non-volatile storage devices) in memory 110, mayinclude a computer readable storage medium. In certain embodiments,memory 110 or the computer readable storage medium of memory 110 maystore one or more of the following programs, modules and datastructures: an operating system that includes procedures for handlingvarious basic system services and for performing hardware dependenttasks; a network communication module that is used for connectingcomputing device 110 to other computers via the one or morecommunication network interfaces and one or more communication networks,such as the Internet, other wide area networks, local area networks,metropolitan area networks, and so on; a client application that maypermit a user to interact with computing device 100.

Certain embodiments of the present invention comprise a trackablehead-mounted display (“HMD”) with at least three degrees of freedom inan operating space and optionally one or more sensors with at least twodegrees of freedom of positional tracking. The HMD and the optionalsensors provide sensory input to a controller, which in turn providessensory feedback to the HMD or another output device. Withoutlimitation, the HMD may be tethered to a stationary computer (such as apersonal computer (“PC”), laptop, or game console), or alternatively maybe self-contained (i.e., with some or all sensory inputs,controllers/computers, and outputs all housed in a single head-mounteddevice).

FIG. 2 is an exemplary diagram of a human user (210) wearing ahead-mounted virtual reality apparatus (220) comprising opticalreceivers and sensors (230 a, 230 b, 230 c, etc.) that may be used toimplement aspects of certain embodiments of the present invention.

FIG. 3 is an exemplary diagram of a transmitter/receiver configurationin an optical positional tracking system that may be used to implementaspects of certain embodiments of the present invention. As depicted inFIG. 3, an exemplary optical positional tracking system comprises a basestation (320) that sweeps an optical signal (310) across the trackingvolume. Depending on the requirements of each particular implementation,more than one base station may be incorporated, and each base stationmay generate more than one optical signal. For example, while a singlebase station is typically sufficient for six-degree-of-freedom tracking,multiple base stations may be necessary in some embodiments to providerobust room-scale tracking for headsets and peripherals. Opticalreceivers (e.g., 230) are incorporated into the head-mounted virtualreality apparatus (220) or other tracked objects. In certainembodiments, optical receivers are paired with an accelerometer andgyroscope Inertial Measurement Unit (“IMU”) on each tracked device tosupport low-latency sensor fusion. As shown in FIG. 3, a standard12-ounce soda or beer car (330) is depicted to provide a sense of scale.

Each base station (320) according to certain embodiments contains tworotors, which sweep a linear beam (310) across the scene on orthogonalaxes. At the start of each sweep cycle, the base station (320) accordingto certain embodiments emits an omni-directional light pulse (“syncsignal”) visible to all sensors. Thus, each sensor computes a uniqueangular location in the swept volume by timing the duration between thesync signal and the beam signal. Sensor distance and orientation issolved using multiple sensors affixed to a single rigid body.

Depending on the particular requirements of each implementation, variousother tracking systems may be integrated, using techniques that are wellknown by skilled artisans.

FIG. 4 is an exemplary diagram of a head-mounted virtual reality display(220) with four optical receivers (230 a, 230 b, 230 c, 230 d) that maybe used to implement aspects of certain embodiments of the presentinvention.

The HMD in certain embodiments presents the user a dynamic virtualenvironment (“virtual space”). It is tracked in operating space so thatits user's motions in the operating space are translated to therepresentation of the virtual space. When the HMD or an optionaladditional sensor close in on physical obstacles in the operating space,such as walls or furniture, a sensory feedback is provided to the usereither in the virtual space or in the operating space in order to avoida collision.

In certain exemplary embodiments the system is primed by the user inadvance by defining the boundaries and limitations of the operatingspace through one or more methods of programmatic input (“Soft Bounds”).In others, the system automatically detects actual obstacles in spacethrough one or more sensor technologies (“Hard Bounds”).

A controller according to aspects of certain embodiments receives andprocesses detection signals from the HMD and/or external sensors andgenerates corresponding feedback to the user based on the proximity ofSoft Bounds and/or Hard Bounds and/or based on explicit user request forfeedback (e.g., an “overlay button”).

Without limitation, definition of Soft Bounds can be made in thefollowing ways (including combinations), depending on the requirementsof each particular implementation:

-   -   By entering data in a computer by means of mouse and keyboard;    -   By moving a tracked object (HMD or other worn sensors such as a        game controller) to the n corners of the operating space; and/or    -   By wearing a tracked HMD in one location, looking (either        physical rotation of the HMD or rotation of the eyeball if the        HMD has gaze-tracking technology) at each of the n corners of        the operating space from at least two places in the operating        space (triangulating the corner that is looked at).

Without limitation, definitions of Hard Bounds can be made in thefollowing ways (including combinations), depending on the requirementsof each particular implementation:

-   -   By using a depth camera attached to the HMD or external to it;        and/or    -   By using other sensors for measuring distances such as lasers or        ultrasound.

Without limitation, sensory feedback can be given in the following ways(including combinations), depending on the requirements of eachparticular implementation:

-   -   a warning sound;    -   haptic feedback (rumble or actuators included in the HMD or        another wearable device such as a game controller held by the        user);    -   visual warning signs displayed in the HMD;    -   a CG-rendered overlay of the Soft Bounds displayed in the HMD        superimposed over the virtual environment;    -   a pass-through video signal of one or more cameras mounted on        the HMD; and/or    -   a custom application program interface (“API”) trigger sent to        the controller of the virtual environment, meaning that the        dimensions of the virtual environment are automatically adjusted        to the physical environment and/or a custom warning specific to        the virtual environment is made (e.g., an in-game character        tells the user to stop walking when the user approaches the        boundaries of the operating space).

Any of the above systems and methods may be implemented either as adigital warning signal that is triggered as a user crosses a predefinedthreshold, or as an analog warning that increases in intensity (e.g.,overlay of room bounds fades in brightness the closer a user gets to anobstacle and gets supported by a rumbling sensation increasing inintensity as user gets even closer).

One embodiment allows the user to dynamically reposition his or hervirtual representation in the virtual space so as to be able toexperience a larger area of the virtual environment than what isprovided by his or her operating space. In one such exemplaryembodiment, the operating space is a 3-meter by 3-meter square. Thevirtual environment is a room several times the size of this operatingspace. In order to experience all of it, a user could trigger areposition of his representation in the virtual environment. In thisexample, the user could move around and rotate a “ghosted” virtualrepresentation of him or herself and a 3-meter by 3-meter squareprojected onto the ground of the virtual environment. Upon accepting therepositioned space, the user would be “teleported” to his or her newplace in the virtual environment and could continue moving around thisnew part of the virtual environment by physically moving in his or heroperating space.

FIG. 5 depicts an exemplary display according to certain embodiments, inwhich soft bounds are visible on the floor as a translucent polygon andhave been defined by the user, and in which hard bounds are visible as agrid of glowing lines that indicate the position of a physical wall in auser's real-life operating space.

FIG. 6 depicts an exemplary display according to certain embodiments, inwhich two in-headset views of hard-bounds chaperoning systems are shown,warning a user of an impending collision, and in which the depictedgrids indicate real-world wall positions.

FIG. 7 depicts an exemplary display according to certain embodiments,depicting an example of using soft bounds data for placement of otherelements in a game scene that help frame the player's experience. In thescene shown in FIG. 7, torches (710 a, 710 b, 710 c, 710 d) areautomatically placed at the bounds corners, regardless of how a user mayhave configured them. The bow (720) shown in FIG. 7 is placed at theleading edge of the user's soft bounds (730), in the user's preferreddirection of play. Certain embodiments according to the arrangementshown in FIG. 7 automatically scale and adjust to a individual user'ssoft-bound settings.

In certain implementations, the appearance of chaperoning boundsaccording to aspects of the present invention may reduce the sense ofimmersion that a user experiences in a virtual environment. This can beaddressed by the following solutions, either separately or incombination, depending on the requirements of each particularimplementation:

First, chaperoning bounds are not displayed at full brightnessimmediately, but instead are slowly faded in as a user closes in on theactual bounds of the user's real environment (“operating space”).Independent fade value may be computed for each wall, then a fifth fadevalue (assuming an exemplary typical operating space in a room with fourwalls) is applied to a perimeter mesh that is the outer edges of thespace in which the user is standing (e.g., this may appear as the edgesof a cube highlighted). The fifth fade value in one embodiment may beimplemented as the maximum value of the fade values for each of the fourwalls. In this way, if a user is backing into a wall, the perimeter meshwill light up full bright. In certain embodiments, to assist a user tosee the other of the walls as the user backs into a wall, the fadevalues may intentionally bleed slightly into their neighboring walls andslightly into the opposite wall. This technique allows a user to see thelocation of all walls without the chaperoning alerts becomingoverwhelming. In certain embodiments, to increase the sense ofimmersion, after the brightest chaperoning bounds are activated anddisplayed at full brightness for some period of time (e.g, 4 seconds),the brightness of all chaperoning alerts is slowly faded to 20% of theoriginal brightness.

Second, only the bounds of the wall closest to user are shown at fullintensity (e.g., as a glowing grid), while the other walls are onlyshown as their outlines/outer corners. Third, the intensity of thechaperoning bounds may be defined relative to the brightness of thevirtual environment they are superimposed on. This underlying brightnesscan either be measured live based on the rendered virtual environment,or provided by the game driving the experience through an API. Fourth,after a user has stood still in one place for a few seconds and hasgotten to understand where the bounds are, chaperone bounds may beautomatically faded out so that user can experience the VR environmentundisturbed in spite of being close to a wall.

In certain implementations, chaperoning alert systems according toaspects of the present invention may show a warning too late for a userto stop before a collision if the user is moving too quickly. This canbe addressed by the following solutions, either separately or incombination, depending on the requirements of each particularimplementation:

First, the chaperoning warnings may be shown earlier intentionally.However, this may have the undesirable effect of making the usable spacein which a user can experience VR smaller. Second, the velocity and/oracceleration of tracked objects (e.g., the user's HMD apparatus and/orrelated handheld controllers) may be measured, and the chaperone boundsmay be shown sooner or later based on the outcome of these measurements.Third, the risk of rapid movement and therefore the speed/intensity ofthe display of chaperoning warnings may be derived from heuristics. Forexample, systems according to aspects of the present invention maymeasure how users generally experience a specific VR experience (e.g.,is it one in which slow exploration is typical, or one in which fastmovement is typical?). Also, if an exemplary system is designed toidentify a user (e.g., by login, eye tracking cameras, height, typicalmotion patterns, etc.) it can base its warnings on how quickly thisparticular user typically moves and reacts to chaperone warnings.Fourth, if a game/application does not actually need a large use space,chaperone warnings can be more aggressive since the need formaximization of space is lower.

In certain implementations, initial room setup according to aspects ofthe present invention may be perceived as relatively manual andunintuitive. This can be addressed by the following solutions, eitherseparately or in combination, depending on the requirements of eachparticular implementation:

First, a user can simply walk around in the real operating space,holding his or her controller and moving it along some or all of thewalls/floor/ceiling of the operating space. The measurements taken viathis process are transmitted to the chaperoning system controller usingany appropriate technique, as known to skilled artisans. Based on theseabsolute measured positions, the systems according to aspects of thepresent invention then calculate the smallest polyhedron that containsall of the positions in which the controller has been detected. Second,rangefinders of various types that are known to skilled artisans (e.g.,ultrasound, laser) may be integrated into particular implementations,and these may generate the necessary information regarding theboundaries of the operating space, with little or no interventionrequired by a user.

In certain embodiments, extending the concept of the independentlycontrolled persistent ground perimeter, wall styles may be separatedfrom perimeter styles, where the perimeter includes the vertical wallseparators, ceiling outline, and ground outline. Perimeter styles couldbe a subset of:

-   -   1. Dynamic perimeter    -   2. Dynamic perimeter with persistent ground outline    -   3. Persistent ground outline only    -   4. Dynamic ground outline only (for the true minimalist who        knows his or her space very well)    -   5. None

In certain embodiments, users may select the invasiveness,aggressiveness, fade distance, and/or color scheme of the chaperoningbounds that are displayed, via any suitable user interface and/orconfiguration utility using techniques that are known to skilledartisans. For example, in terms of color scheme selection, a suitablepalette of colors may be predetermined from which a user may select, orusers may be permitted to choose hue and/or saturation, while brightnessis generated by systems according to aspects of the present invention.Moreover, user selections may be adjusted and/or saved, depending onparticular games or applications.

FIG. 8 depicts an exemplary display according to certain embodiments,depicting an example of rendering a square pattern on the walls of avirtual space to visually alert a user as to the location of physicalboundaries in the real operating space surrounding the user.

FIG. 9 depicts an exemplary display according to certain embodiments,depicting another example of rendering a grid pattern on the walls of avirtual space (with the intersections removed) to visually alert a useras to the location of physical boundaries in the real operating spacesurrounding the user.

FIG. 10 depicts an exemplary display according to certain embodiments,depicting another example of rendering a pattern on the walls of avirtual space to visually alert a user as to the location of physicalboundaries in the real operating space surrounding the user. This issimilar to the pattern shown in FIG. 9, but the square openings areapproximately 2.25 times greater in area (i.e., the bars are spaced 1.5times farther apart).

FIG. 11 depicts an exemplary display according to certain embodiments,depicting another example of rendering a pattern on the walls of avirtual space (i.e., a single horizontal line on each wall) to visuallyalert a user as to the location of physical boundaries in the realoperating space surrounding the user.

FIG. 12 depicts an exemplary display according to certain embodiments,depicting an example of rendering a pattern on the floor of a virtualspace to visually alert a user/developer as to the location of physicalboundaries in the real operating space surrounding the user. Dependingon the requirements of each particular implementation, this may bedisplayed as a persistent line loop at floor-level that never fadesaway, to assist a user or developer to always be aware of where thewalls are located within the operating space simply by glancing down.

While the above description contains many specifics and certainexemplary embodiments have been described and shown in the accompanyingdrawings, it is to be understood that such embodiments are merelyillustrative of and not restrictive on the broad invention, and thatthis invention not be limited to the specific constructions andarrangements shown and described, since various other modifications mayoccur to those ordinarily skilled in the art, as mentioned above. Theinvention includes any combination or sub-combination of the elementsfrom the different species and/or embodiments disclosed herein.

We claim:
 1. A method for warning a user of a head-mounted display ofpotential collisions with real-world obstacles, comprising the steps of:defining the location of said obstacles relative to a reference point;continually monitoring a location of said user relative to saidreference point; performing a first detection to determine when a firstdistance between said location of said user relative to a location ofone of said obstacles is smaller than a first predetermined alarmthreshold; in response to said first detection, displaying a visualalarm on said head-mounted display to provide an indication to said userof a potential collision with said one of said obstacles; andcontinually monitoring a location of an object held by said user,performing a second detection to determine when a second distancebetween said location of said object relative to said location of saidone of said obstacles is smaller than said first predetermined alarmthreshold, and, in response to said second detection, displaying avisual alarm on said head-mounted display to provide an indication tosaid user of a potential collision between said object and said one ofsaid obstacles, wherein a step of defining said location of saidobstacles relative to said reference point further comprises promptingsaid user to walk around a portion of the boundaries of a real operatingspace while holding said object, and detecting said location of saidobject at a plurality of points in time during an execution of saidstep.
 2. The method of claim 1, wherein, if said first detectiondetermines that the first distance between said location of said userrelative to said location of said one of said obstacles is smaller thansaid first predetermined alarm threshold, said visual alarm increases inintensity as said user gets closer to said one of said obstacles.
 3. Themethod of claim 2, wherein said visual alarm is accompanied by avibratory alarm on said object if said second detection determines thatthe second distance between said location of said object relative tosaid location of said one of said obstacles is smaller than a secondpredetermined alarm threshold.
 4. The method of claim 1, wherein, ifsaid second detection determines that the second distance between saidlocation of said object relative to said location of said one of saidobstacles is smaller than said first predetermined alarm threshold, saidvisual alarm increases in intensity as said object gets closer to saidone of said obstacles.
 5. The method of claim 1, wherein said visualalarm is accompanied by a vibratory alarm on said head-mounted displayif said first detection determines that the first distance between saidlocation of said user relative to said location of said one of saidobstacles is smaller than a second predetermined alarm threshold.
 6. Themethod of claim 1, further comprising the step of detecting an issuanceof a repositioning command from said user and, in response,repositioning a virtual representation of said user in a virtual space.7. The method of claim 1, further comprising reducing a brightness ofsaid visual alarm after a predetermined period of time.
 8. A system forwarning a user of a head-mounted display of potential collisions withreal-world obstacles, comprising: one or more location-defining circuitsfor determining the location of said obstacles relative to a referencepoint; one or more circuits for user-location monitoring thatcontinually monitor a location of said user relative to said referencepoint, perform a first detection to determine when a first distancebetween said location of said user relative to a location of one of saidobstacles is smaller than a first predetermined alarm threshold, and inresponse to said first detection, display a visual alarm on saidhead-mounted display to provide an indication to said user of apotential collision with said one of said obstacles; and one or morecircuits for object-location monitoring that continually monitor alocation of an object held by said user, perform a second detection todetermine when a second distance between said location of said objectrelative to said location of said one of said obstacles is smaller thansaid first predetermined alarm threshold, and, in response to saidsecond detection, display a visual alarm on said head-mounted display toprovide an indication to said user of a potential collision between saidobject and said one of said obstacles, wherein said one or morelocation-defining circuits prompt said user to walk around a portion ofthe boundaries of a real operating space while holding said object, anddetect said location of said object at a plurality of points in time assaid user walks around said portion.
 9. The system of claim 8, wherein,if said first detection determines that the first distance between saidlocation of said user relative to said location of said one of saidobstacles is smaller than said first predetermined alarm threshold, saidvisual alarm increases in intensity as said user gets closer to said oneof said obstacles.
 10. The system of claim 9, wherein said visual alarmis accompanied by a vibratory alarm on said object if said seconddetection determines that the second distance between said location ofsaid object relative to said location of said one of said obstacles issmaller than a second predetermined alarm threshold.
 11. The system ofclaim 8, wherein, if said second detection determines that the seconddistance between said location of said object relative to said locationof said one of said obstacles is smaller than said first predeterminedalarm threshold, said visual alarm increases in intensity as said objectgets closer to said one of said obstacles.
 12. The system of claim 8,wherein said visual alarm is accompanied by a vibratory alarm on saidhead-mounted display if said first detection determines that the firstdistance between said location of said user relative to said location ofsaid one of said obstacles is smaller than a second predetermined alarmthreshold.
 13. The system of claim 8, further comprising one or morecircuits for display repositioning that detect an issuance of arepositioning command from said user and, in response, reposition avirtual representation of said user in a virtual space.
 14. The systemof claim 8, wherein said one or more circuits for user-locationmonitoring reduce a brightness of said visual alarm after apredetermined period of time.